A pyrometer is a device that intercepts and measures thermal radiation in a non-contact temperature sensing process known as pyrometry. A typical pyrometer has an optical system and detector; the optical system focuses thermal radiation onto the detector. The detector produces an output signal, typically a voltage, which is related to thermal radiation, or iridescence, of a target object. Therefore, the output signal of the detector can be used to infer the temperature of the target object, while negating the need for direct contact between the temperature detecting device and the target object.
The thermal radiation of an object depends on its emissivity. Emissivity is the property of a material's surface that describes its “efficiency” at emitting thermal radiation. An emissivity value of 1.0 represents thermal radiation emission at 100% while an emissivity of 0 describes thermal radiation emission at 0% (or perfect reflection).
Typically, for non-metals and coated metals emissivity is very high, 0.8 or greater, and variations in emissivity are less of a problem for non-contact temperature detection. For example, a production process in which a non-metallic material with an emissivity of 0.9 is to be temperature-controlled, and if normal material variations cause emissivity variations of ±0.01, the associated temperature error will be of the order of 0.01 divided by 0.9, or about 1% of the temperature reading, an acceptable variation for many applications. In contrast, for a production process in which the temperature of a metal having an emissivity of 0.2 must be controlled, emissivity variations of ±0.01 will produce an error on the order of 0.01 divided by 0.2, or about 5% of the temperature reading, which is typically unacceptable. Additionally, metal finishes, which play a significant role in emissivity, tend to have more variations than non-metals. A common problem is aluminum because its emissivity is low and variable due to alloying, surface oxidation, variations in surface finish, and other factors.
Ratio pyrometers typically use two photo sensors to detect radiation at two separate wavebands. Temperature can be determined by taking the ratio of the detected radiation. Traditional ratio pyrometers, which date from about the middle of the last century, operate with two narrow spectral bands. As such, they are successful at measuring targets of strong radiance. In other words, they are useful for measuring high temperature targets. However, for targets with lower temperatures, and therefore a lower radiance, the narrow spectral bands receive insufficient photo signals, rendering the technique useless.
U.S. Pat. No. 5,764,684, titled “Infrared Thermocouple Improvements,” issued Jun. 9, 1998, discloses a device and method which employs infrared sensors with very wide bandwidths, thereby increasing the radiation detector output, such that relatively low temperatures (i.e., less than 50° Celsius (C.)) can be measured. Further disclosed is the side-by-side placement of a short wavelength low emissivity infrared (IR) thermocouple and a long wavelength infrared thermal couple focused on the same target area at a particular distance. The two thermocouple input channels provide input to a computer or PLC. The computer or PLC has the computational ability to solve two equations with two unknowns, with one such solution being the computation of differentials in signal relative to the initial calibration. This solution depends on the assumption that the emissivity ratio remains constant for the two wavebands of interest.